In a wireless telecommunications system, radio channels provide a physical link between communications units. The equipment in such a system typically includes a base station processor in communication with a network such as the Public Switched Telephone Network (PSTN), in the case of voice communications, or a data network, in the case of data communications, and one or more access terminals in communication with a plurality of end user computing devices, such as user PCs. The combination of access terminal and end user computing device may be referred to as a field unit or remote units. The wireless channels include forward channels, for message transmission from the base station processor to the field units, and reverse channels, for message transmission to the base station processor from the field units.
In the case of a wireless data system such as may be used to provide wireless Internet access, each base station processor typically serves many field units. The wireless channels, however, are a limited resource, and are therefore allocated by a scheduler among the field units served by the base station processor. The scheduler allocates the wireless channels among the field units on a traffic demand basis.
One way of supporting on-demand access among multiple users is referred to as Time Division Multiple Access (TDMA), where each of the wireless channels are allocated to specific access terminals only for a certain number of predetermined time intervals or time slots. A second way of supporting on-demand access among multiple users is referred to as Code Division Multiple Access (CDMA), which allows multiple users to share the same radio spectrum. Instead of dividing a Radio Frequency (RF) spectrum into narrow channels (e.g. 30 kHz each in analog wireless systems), CDMA spreads many channels over a broad spectrum (1.25 MHZ in the case of the North American CDMA standard known as IS-95). To separate a particular channel from another channel using the same spectrum at the same time, a unique digital code called a pseudo-random (i.e., pseudo-noise or PN) code is assigned to each user. Many users (up to 64 for IS-95) share the same spectrum, each using their unique code, and decoders separate the codes at each end in a process similar to a tuner that separates different frequencies in more conventional systems.
The PN codes used for communication channel definitions typically have a defined code repeat period or code epoch. For each such epoch duration (also called a slot), a base station central controlling system can further schedule assignments of forward traffic channels (forward slot allocations or “FSAs”) and reverse traffic channels (reverse slot allocations or “RSAs”) to active field units for each epoch. This is typically done in such a way that all channels are assigned to active users as much as possible. Unfortunately, the need to assign and reassign PN code channels among a large number of users can introduce delays. In particular, when a PN code is reassigned to a different user connection, it typically takes a determined period of time for the code demodulators in the receiver to lock in the new code. This in turn introduces latency in the reception of the data packets that must travel on the coded channel.
To coordinate traffic channels, the base station processor communicates with a given field unit in the following manner. First, the base station processor checks to make sure there is an available channel. Second, the base station processor sends a message to the given field unit to set up the available channel. The given field unit processes the message (2-3 slots) to set-up the channel and sends an acknowledgment (1-2 slots) confirming set-up complete. To tear down the channel, the base station processor sends a message to the given field unit, which processes the command (1-2 slots) and sends back an acknowledgment (1-2 slots).